Apparatus for detecting the thickness of sheets

ABSTRACT

Apparatus for detecting the presence of a single banknote or a double banknote (5) comprises a cylindrical roller (1) biased by way of a pivot arm (2) against a moving surface (6) which conveys banknotes under the roller (1). The other end of the pivot arm is connected to the core of a linear variable differential transformer (4) which is activated by an oscillator (7). The output of the L.V.D.T. (4) is proportional to the displacement of the roller (1) normal to the surface (6) and is thus indicative of the thickness of any banknotes passing under the roller. In order to compensate of mechanical and electrical drift in the thickness level corresponding to the absence of a banknote, the auto-reference circuit (9) derives first and second threshold values for single and double notes respectively by adding predetermined potentials to the reference potentials. These predetermined differentials may be controlled manually by a variable resistor (11) within a thickness control circuit (10).

BACKGROUND OF THE INVENTION

This invention relates to apparatus for detecting the thickness ofsheets as they are moved past the apparatus, and is especiallyapplicable to the detection of banknotes where the thickness of two ormore superimposed banknotes should be distinguished from that of asingle banknote.

Machines for detecting the simultaneous passage of two banknotes wherebanknotes should normally pass singly are known, and one example of anoptical detector is in commonly assigned British Pat. No. 1,344,986, thedetector responding to the opacity of the note or notes; this isinadequate if the notes are greasy or soiled. Mechanical thicknessdetectors are also known using a roller whose axis is displaced by thepassage of a banknote or banknotes to cause an electrical contact to bemade or broken if the displacement is such as to indicate the passage ofsuperimposed banknotes. Apparatus for gauging laminar materials, usingnip rollers and a variable-reluctance transducer, is disclosed incommonly assigned British Pat. No. 1,497,181.

However, none of the machines has yet been able to compensate forlong-term variations in the detecting heads and in the control circuitrywhich will tend to change the threshold value of the banknote thickness,i.e. the thickness at which the detector will signal the passage of adouble banknote. In the case of the mechanical detector using a roller,this long-term drift could be caused by mechanical movement between thepivot of an arm attached to the axis of the roller, and the surface onwhich the note is placed as it is moved under the roller. Thispotentially degrades note thickness measurement if the output of thedetector is compared against a fixed threshold level in the outputstages of the control circuit. Mechanical stability of the orderrequired for measuring banknote thicknesses, for example 0.005 cm to0.015 cm, is difficult to achieve under reasonable manufacturing andwear rate conditions.

SUMMARY OF THE INVENTION

Apparatus according to the invention for the detection of single ormultiple sheets comprises: a support on which the sheet or sheets iscarried for movement in the plane of the sheets; a follower biased suchthat it normally abuts against the support and follows the profile ofthe surface of the support and the overlying sheet or sheets; atransducer responsive to the position of the follower to provide athickness signal indicative of the linear displacement of the followerrelative to the support due to the passage of the sheet or sheets; and acontrol circuit which compares the thickness with at least one storedthreshold level signal to provide an output signal indicative of whetherthe detected thickness corresponds to that of a single sheet or multiplesheets; and characterized in that the control circuit includes means forrepeatedly updating the or each stored threshold level signal from thethickness signal provided when there is no sheet between the support andthe follower. There may be an arm for conveying the movement of thefollower to the transducer. This can be a pivot arm, the follower beinga cylindrical roller whose axis is attached to the end of the pivot arm.The position of the pivot point along the arm is then preferably suchthat small displacements of the follower are converted into largerdisplacements of the other end of the arm engaging with the transducer.The transducer is then ideally a linear variable differentialtransformer.

An alternative apparatus comprises a follower which is a cylindricalroller, the transducer being of variable reluctance and comprising atransformer whose magnetic circuit is completed by a portion of theperiphery of the said follower, the thickness signal being the voltageacross the transformer which depends on the degree of displacement ofthe follower roller towards a core of the transformer.

The control circuit preferably includes a manually adjustable levelcontrol by means of which the expected thickness of a single sheet isdesignated, the control circuit then deriving at least a first thresholdlevel, for the thickness of a single sheet, from the said expectedthickness and from the reference signal provided by the transducer whenthere is no sheet between the surface and the follower.

The control circuit preferably also derives a second threshold level forthe thickness of a double sheet, and the output signal indicates thepassage of a single or a double sheet. The apparatus may be morecomplex, and is capable of being extended so as to indicate the passageof three or more superimposed sheets, or else to indicate differenttypes of sheets passing under the roller, the different types havingdifferent thicknesses.

BRIEF DESCRIPTION OF THE DRAWINGS

Two embodiments of the invention will now be described with reference tothe accompanying drawings, in which:

FIG. 1 illustrates diagrammatically apparatus for detecting thethickness of banknotes and an associated control circuit according to afirst embodiment of the invention;

FIG. 2 shows the transducer of FIG. 1 in greater detail;

FIGS. 3A and 3B, which are halves of the same circuit diagram, show thecontrol circuit in greater detail;

FIG. 4 is a waveform diagram for signal TP3 of FIG. 3; and

FIG. 5 illustrates apparatus according to a second embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1, a banknote 5 is moved by a conveyor 6, whosesurface also serves as a reference datum, into the nip formed between acylindrical roller 1 and the conveyor surface. The roller 1 is attachedat its axle to a pivot arm 2 which is biased in the anticlockwisedirection but is free to rotate about a pivot 3. A linear transducerconstituted by a linear variable differential transformer (LVDT) 4 has amovable core attached to the other end of the pivot arm and is energisedby the oscillator 7. The downward displacement of the core when theroller is pushed up by the passage of the banknote under the roller isproportional to the thickness of the banknote, and the resulting changein the output of the transformer is detected by the level detector 8. Asignal having an amplitude corresponding to the thickness of thebanknote is fed by the level detector 8 to an auto-reference circuit 9.An alternative to this arrangement would be to dispense with the pivotarm 2 and to drive the LVDT core directly, e.g. by a jockey wheel.

The expected thickness of a banknote is set manually by a control 11 fora thickness control circuit 10, which sends a signal to theauto-reference circuit 9. The auto-reference circuit derives a thresholdlevel for a single banknote, preferably by adding the expected thicknessto a reference datum level provided by the level detector 8. Theauto-reference circuit also derives the threshold level for a doublebanknote. During operation of the apparatus, the auto-reference circuitrepeatedly updates these threshold level values when there is nobanknote under the roller, to take into account changes in the datumlevel.

When one or more banknotes pass under the roller, the level detector 8provides a thickness signal which is compared in the comparators 12 and13 with the threshold values, which in turn provide output signals 14and 15 according to the results of the comparisons.

The thickness control 11 can be manually adjusted by an operator, forexample, when changing from feeding British banknotes to Frenchbanknotes which are notably thinner than the British notes.

Referring to FIG. 2, the end of the pivot arm 2 which engages with theL.V.D.T. 4 is attached via a short rod to a core 24, which is free tomove axially within the L.V.D.T. Movements of the roller 1 are convertedby the pivot arm into larger, anti-parallel movements of the core 24,and the axial position of the core determines the magnitude of thesignal provided by the L.V.D.T. to the level detector 8.

The L.V.D.T. 4 is symmetrical about a central plane C-C normal to theaxis, and the upper half (in FIG. 2) is not used. With the roller 1touching the surface 6, with no note present, the core 24 should be at aposition Q-Q, a little displaced from the central plane. This is knownas the quiescent start point. The L.V.D.T. provides a zero output withthe core 24 at the central plane C-C, but provides signals of increasingmagnitude as the core is moved away from this plane (in eitherdirection). With a single note thickness between the roller 1 and thesurface 6, the core 24 reaches a plane S-S, and with a double thicknessit reaches a plane D-D. In use, atmospheric variations and general wearmay cause these positions and the quiescent start point to drift ineither direction, it being the purpose of the auto reference circuit tocompensate for this drift. The acceptable limit of drift in onedirection for the quiescent start point Q-Q is the central plane C-C;the region of drift in this direction is the lower drift area LDA. Theacceptable limit of drift in the opposite direction for the doublethickness plane D-D is the end of the transformer coil, beyond whichthere is no longer a linear relationship between the core position andthe output signal; the region of drift in this opposite direction is theupper drift area UDA.

For the system to cope with the maximum range of mechanical drift, thesensor core must be positioned at the optimum quiescent start pointwithin the sensor body when the system is fitted to a machine.

The quiescent start point can easily be set by adjusting the sensor bodyto give a reference voltage output from the level detector 8 to the autoreference circuit 9 of 0.5 volts d.c.

There are, however, two such positions, corresponding to reflections ofthe core position about the central plane C-C, or sensor zero-outputpoint. The correct one can be found by moving the core 24 in thedirection of increasing banknote thickness and checking that the voltageoutput also increases. With the L.V.D.T. and core correctly adjusted,the voltage output from the level detector 8 is the 0.5 volt referencevalue with no note present, and, for example, 0.8 volt with a singlenote and 1.1 volts with a double note. These values will be referred toas TP3, TP1 and TP2 respectively. A convenient way of adjusting theL.V.D.T. to give an output of 0.5 volt with no note present is by meansof a voltage monitoring circuit, as shown at the bottom of FIG. 3B. Thevoltage signal TP3 is fed into the monitoring circuit, which lightseither a red light-emitting diode LED1 or a green one LED 2 depending onwhether the voltage TP3 is outside or inside acceptable voltagetolerances around the nominal 0.5 volt level. A voltage divider networkof resistors RR1 (e.g. 5.K6), RR2 (e.g. 220Ω) and RR3 (e.g. 560Ω)connected between the reference 5 volt level and ground provides avoltage slightly above 0.5 to a first comparator CC1, and a voltageslightly below 0.5. volt to a second comparator CC2. The signal TP3 isfed to the other input terminal of each comparator, and the outputs ofthe comparators are added to produce a signal which is at one level ifthe signal TP3 is close to 0.5 volt, and another level if it is outsidethis tolerance. This signal is fed to a pair of series-connected pnp andnpn transistors, so connected that either LED1 or LED2 energizesdepending on the signal value. The L.V.D.T. core is moved relative tothe transformer until the green light LED2 comes on.

FIGS. 3A and 3B are two halves of the circuit diagram of this preferredembodiment of the invention.

The L.V.D.T.4 is driven by a primary drive oscillator IC3/1 which runsat about 10 kHz. The whole oscillator circuit 7 supplies an outputthrough an electrolytic capacitor to the drive coil of the L.V.D.T.4.(Throughout FIGS. 3A and 3B, the symbol "REF" indicates a connection toa reference voltage source of 0.5 volts, being the same level as theoutput TP3 for no note present under the roller 1.) The L.V.D.T. sensoroutput is amplified about fives times by amplifier IC3/2, and reachesdiode D7 via a high pass filter, which allows only the carrier contentof the signal to pass through. Diode D6 provides D.C. restoration togain full advantage of the envelope.

The signal emerging from diode D7 is labelled TP and is of the formshown in FIG. 4. This waveform has a first peak corresponding to asingle note, a second peak for a double note, and third peak similar tothe first for another single note. The lowest level of signal TP isnominally at 0.5 volt, the reference level for TP3. The single- anddouble-note peaks exceed their respective thresholds TP1 and TP2.

The output signal TP from the level detector 8 is fed to the autoreference circuit 9, which is connected to circuitry 12, 13 fordetermining the presence of a single or a double note. Signal TP is fedto an amplifier IC2/1 whose output normally follows the TP signal. Whena note appears, a transistor TR6 connected between the output of IC2/1and the reference level, is switched off by a single note comparator 12(IC2/3) connected to the base of transistor TR6. This prevents theoutput of IC2/1 from following the level of the output signal TP while anote is present.

The output level of IC2/1 is maintained by an electrolytic capacitorC13, connected between the output of IC2/1 and ground. This level ismaintained by the capacitor C13 until the note has passed. A resistornetwork consisting of resistors R48, R50 and R51 ensures that thecapacitor C13 always attempts to charge towards the optimum quiescentpoint to prevent any possible locking out of the system. The outputlevel from the amplifier IC2/1 is also conveyed along line L to an inputof another amplifier IC2/2, shown in the upper half of FIG. 3A. Thispart of the circuit is responsible for obtaining reference levels TP1and TP2 from the reference defined by the potential stored in capacitorC13. The reference levels for single and double notes are derived fromthe reference level for when no note is present. The purpose of thispart of the circuit is to maintain the differences between the referencelevels TP1, TP2 and TP3 despite any variations in the reference levelTP3, the differences being determined by a thickness control circuit 10comprising a variable resistor RV1. An amplifier IC3/3 and a transistorTR8 are connected between the reference potential and ground so as toconstitute a constant current source. The current output of thisconstant current source is determined by the value of the variableresistor RV1 (11), in accordance with the desired threshold thicknesslevel. Reference resistors R49 and R52 are connected in series betweenthe collector of transistor TR8 and the output of the amplifier IC2/2.The reference resistors R52 and R49 are fed by the constant currentsource TR8, IC3/3 and the resistors together with the constant currentsource form a transconductance amplifier. The current passing throughthe reference resistors sets up the two reference thresholds TP1 and TP2for single note and double note respectively. The reference voltages arefixed above the output of amplifier IC2/2 which is effectively the TP3"no note" level. Any change in the reference level or no note, TP3,therefore causes the reference levels TP1 and TP2 to change by the sameamount, thus retaining the reference differentials. The references forsingle and double thicknesses of notes are therefore always made withreference to the current quiescent displacement of the core of theL.V.D.T. sensor 4.

The reference voltage TP1 and TP2 are conveyed to difference amplifiersIC2/3 and IC2/4 respectively, as shown in FIG. 3B. Amplifiers IC2/3 andIC2/4 together with their respective feedback resistors constituteoutput comparators for single and double notes respectively, designated12 and 13 in FIG. 1. The other input to the comparators in each case isthe current value of the output TP from the level detector 8. Theoutputs 15, 14 of the output/comparators 12, 13 are fed to transistorsTR3 and TR4, the emitter of each transistor being connected to thereference potential. Transistors TR3 and TR4 are normally-saturatedswitches. Transistor TR3 turns off when a single note is present, andTR4 turns off when a double note is present. These transistors provideoutputs at points 31, 32 and 33 to further circuitry (not shown) whichmay respond to the presence or absence of single or double notes.

In the circuit shown in FIG. 3A, reference resistors R49 and R52 neednot have the same value, thus allowing the differentials between thethreshold potential values to be different. The difference between TP2and TP3 need not be exactly double that between TP1 and TP3.

The control circuit described above responds to the presence of singleor double banknote thicknesses, but could be extended to respond tomultiple banknote thicknesses, or to the thicknesses of different typesof banknotes. The detector could for example discriminate between athick note and a thin note and/or between double thickness of each typeof note.

Apparatus according to a second embodiment of the invention is shown inFIG. 5, which includes nip rollers and a variable reluctance transducerof the type discloed in British Pat. No. 1497181. The banknote 5 is fedinto the nip of rollers 51 and 52. Roller 51 has an axle which is notfree to move vertically, but follower roller 52 is free to movevertically in accordance with the thickness of the banknote. Thefollower roller is a rubber flexing roller which is eccentricallydeformable and has a steel tyre which moves in the mouth of the magneticcore of a transducer transformer 53, causing the transducer reluctanceto vary in accordance with the gap between the core and the tyre. Ameasurement of the reluctance therefore reveals the thickness of thebanknote.

A coil L in the transformer is supplied with a current i from a currentsource 54, at a voltage Vs which constitutes the a.c. thickness signalprovided to the level detector 8 of FIG. 1. The voltage Vs isproportional to LI.2πf, where L=N² /Rm, L being the inductance of thecoil, N being the number of turns, Rm being the reluctance; I being theamplitude and f being the frequency of the a.c. current i=I sin (2π ft).

This system offers the best possible maintenance-free operation sincethere are no mechanical linkages, and it is unaffected by dust. Also,because the signal is compatible with the control circuit above,mechanical tolerances and long term drift such as wear or thermalexpansion etc. are coped with.

I claim:
 1. Apparatus for detecting the passage of single and multiplesheets (5) along a flow path, comprising: a surface (6) in the flow pathfor supporting the sheet or sheets; a follower (1 or 52) biased suchthat it normally abuts against the supporting surface and follows theprofile of the supporting surface and the overlying sheet or sheets; atransducer (4 or 53) responsive to the position of the follower toprovide a thickness signal (TP) indicative of the linear displacement ofthe follower relative to the supporting surface due to the passage ofthe sheet or sheets; a control circuit (FIGS. 3A, 3B) which compares thethickness signal (TP) with at least one stored threshold level signal toprovide an output signal (31, 32, 33) indicative of whether the detectedthickness corresponds to that of a single sheet or multiple sheets; thecontrol circuit including: means (9) responsive to the transducer outputsignal in the absence of a sheet for repeatedly updating a storedno-sheet signal level; adjustable threshold level generating means (10)including adjustable means for generating first and second signals (TP1,TP2) which correspond to changes in the output of the transducer inresponse to the passage of a single sheet and two superimposed sheets,respectively; and means connecting the first and second signalgenerating means to the output of the updating means to add the updatedno-sheet signal level to the first and second signals to obtain updatedfirst and second threshold levels which will be exceeded by the outputof the transducer in response to the passage of a single sheet and twosuperimposed sheets, respectively.
 2. Apparatus in accordance with claim1, wherein the means for generating the first and second signalscomprises a manually adjustable variable resistor (RV1), a constantcurrent source (IC3/3, TR8) whose output is determined by the variableresistor, and first (R52) and second (R49) reference resistors connectedin series between the output of the constant current source and theoutput of the updating means.
 3. Apparatus in accordance with claim 1,wherein the follower is attached to one end of an arm (2), and thetransducer engages with the other end of the arm.
 4. Apparatus inaccordance with claim 3, wherein the arm is an elongate arm pivoted at apoint (3) along its length for conveying the movement of the follower tothe transducer.
 5. Apparatus in accordance with claim 4, wherein theposition of the pivot point along the arm is such that smalldisplacements of the follower are converted into larger displacements ofthe other end of the arm engaging with the transducer.
 6. Apparatus inaccordance with claims 3, 4 or 5, wherein the follower is a cylindricalroller whose axle is attached to one end of the arm.
 7. Apparatus inaccordance with claim 1, wherein the transducer is a linear variabledifferential transformer.
 8. Apparatus in accordance with claim 1wherein the follower (52, FIG. 5) is a cylindrical roller, thetransducer (53, FIG. 5) being of variable reluctance and comprising atransformer whose magnetic circuit is completed by a portion of theperiphery of the follower (52), the thickness signal being the voltage(Vs) across the transformer which depends on the degree of displacementof the follower roller towards a core of the transformer.
 9. Apparatusin accordance with claim 8, wherein the supporting surface for the sheetor sheets is a second roller (51), which together with the followerconstitutes a nip for the sheet or sheets.
 10. Apparatus in accordancewith claims 8 or 9, wherein the follower is a steel-surfaced,rubbercored roller.